This invention relates to improvements in dry etching method for amorphous carbon used as an organic hard mask.
In today's semiconductor device fabrication, the trend toward micro-patterning for circuits has been developed, with which a photoresist layer used as an etching mask for forming a pattern by exposure has been getting thinned. However, the thus thinned photoresist layer was sometimes insufficient in etching resistance to form a pattern onto an object to be etched. This problem gets particularly outstanding in the case of forming a pattern having a high aspect ratio (a ratio between the dimension and the depth of the pattern). Accordingly, a multilayer resist process where a pattern of a photoresist layer is firstly transferred to another thick lower portion and then the pattern is formed onto an object to be etched by using the thick lower portion as a mask has been adopted in recent years.
An example of the multilayer resist process will be explained with reference to FIG. 2. FIG. 2 illustrates a case intending to provide a hole or line pattern with high aspect ratio on a silicon-based foundation layer 3, for example. First of all, a foundation layer 3 (an object to be etched) is formed on a substrate 2 and then an amorphous carbon layer (a-C layer) 4, an inorganic intermediate layer 5 and a photoresist layer 6 are sequentially laminated as shown in FIG. 2A. Then, the photoresist layer 6 is exposed to light by using a photomask (though not shown) and then developed thereby being provided with a certain hole pattern as shown in FIG. 2B. Thereafter, the inorganic intermediate layer 5 is subjected to etching by using the photoresist layer 6 as a mask to transfer the hole pattern of the photoresist layer 6 to the inorganic intermediate layer 5 as shown in FIG. 2C. Then, the a-C layer 4 is etched by using the inorganic intermediate layer 5 as a mask so that the hole pattern of the inorganic intermediate layer 5 (namely, the hole pattern of the photoresist layer 6) is transferred to the a-C layer 4 as shown in FIG. 2D. Subsequently, the foundation layer 3 is etched by using the a-C layer 4 as a mask so that the certain pattern is formed onto the foundation layer 3 as shown in FIG. 2E. Finally, the masks such as the a-C layer 4 are removed thereby obtaining a foundation layer 3 having a certain pattern, as shown in FIG. 2F. Hence the a-C layer 4, the inorganic intermediate layer 5 and the photoresist layer 6 are sometimes referred to together as a multilayer resist film.
In a method disclosed by Patent Document 1, a multilayer resist mask is provided including an upper resist film (corresponding to the photoresist layer 6 as shown in FIG. 2), an inorganic intermediate film (corresponding to the inorganic intermediate layer 5 as shown in FIG. 2) and a lower resist film (corresponding to the a-C layer 4 as shown in FIG. 2), and these films are processed by different steps or by different etching gases. More specifically, the upper resist film is exposed and patterned through the lithography technique. The inorganic intermediate film is etched by plasma in use of a dry etching agent comprising a mixed gas containing SF6 and CHF3 with the upper resist film 201 as a mask. Then, the lower resist film (serving as an organic film) is etched by plasma in use of a mixed gas containing O2, HBr, and N2 with the inorganic intermediate film as a mask.
With regard to a photoresist constituting the photoresist layer 6 and a silicon-based material constituting the inorganic intermediate layer 5 (which is SiON in most cases), already-existing exposure apparatuses and dry etching apparatuses may be diverted thereto as they are.
However, it is difficult to say that a method authentic as an etching method for amorphous carbon constituting the a-C layer 4 has been established.
This is because isotropic etching tends to occur or because side etch as shown in FIG. 3 tends to occur, in a case of etching amorphous carbon upon converting O2 (which is the principal etching agent) into plasma. As a result, the cross-sectional shape tends to be a round one, the so-called bowing shape. In FIG. 3 where the a-C layer 4 is formed on the foundation layer 3 and additionally the inorganic intermediate layer 5 (serving as an inorganic hard mask) having a certain opening is formed thereon, the a-C layer 4 is scraped more than intended to generate a side etch 7 since isotropic etching is caused at the time of etching the a-C layer 4.
In order to prevent these problems, there are employed for etching the amorphous carbon layer: a mixed gas of O2 and COS, obtained by adding COS (carbonyl sulfide) to O2 (see Patent Document 2); or a mixed gas of O2, COS and Cl2 (see Patent Document 3).